The present invention relates to data compression and watermarking.
In computing, a watermark is typically a bit-sequence that relates to or is associated with other digital content. Unlike printed watermarks consisting of specially printed images or embossed papers, digital watermarks may also correspond to general number sequences, checksums or any other type of digitally transmitted data. Resilient digital watermarks can be used to identify the owner of certain content. These watermarks continue to identify a particular owner despite attempts by an unauthorized third party to remove or alter the content. In contrast, fragile watermarks become unreadable or corrupt when digital content and the underlying watermark are altered. These fragile watermarks ensure the authenticity of certain documents and prevent data tampering from going unnoticed. No doubt, many other uses of digital watermarks exist and will be created in the future.
In most cases, the watermark is transmitted along with an input stream of digital data. If the input stream is not compressed, the watermark can be added to the digital data input stream thereby increasing the overall amount of data being transmitted. Often however, it is desirable to compress this digital data stream thereby increasing the effective network bandwidth during transmission and decreasing storage requirements. One solution is to devise a compression routine that both compresses the input stream and embeds a watermark bit-sequence associated with the watermark.
Adding a watermark to compressed digital data is not particularly difficult when using lossy compression techniques. These lossy compression techniques selectively eliminate certain bits and replace them with the watermark values, or otherwise alter the encoded data to introduce the watermark information. Data loss in the original data stream is caused not only by the lossiness associated with the compression operation but also by the addition of the watermark.
Unfortunately, lossy watermark embedding is particularly unacceptable to users when the underlying compression is lossless. The amount of loss introduced depends a great deal on the specific digital data being compressed and therefore is not easy to predict or limit. Consequently, many lossy techniques for embedding a watermark introduce undesirable distortion to the image as by-product. Users using popular lossless compression methods such as JPEG-LS and its variants expect to have watermark embedding routines available that do not perceptibly distort their images. Moreover, even if the watermarks could be introduced with little or no loss, it is a challenge to ensure that an enhanced decompressor is backward compatible with existing coders and compressed streams. Lack of backward compatibility is one of the more considerable impediments to enhancing lossless compression operations like JPEG-LS to accept watermarks.
Like reference numbers and designations in the various drawings indicate like elements.